Obtaining a History of the Flux of Cosmic Rays using In Situ Cosmogenic 14C Trapped in Polar Ice

Carbon-14 (C-14) is produced in the atmosphere when neutrons from cosmic-ray air showers are captured by N-14 nuclei. Atmospheric C-14 becomes trapped in air bubbles in polar ice as compacted snow (firn) transforms into ice. C-14 is also produced in situ in ice grains by penetrating cosmic-ray neutrons and muons. Recent ice core measurements indicate that in the (CO)-C-14 phase, the C-14 is dominated by the in situ cosmogenic component at most ice coring sites. Thus, it should be possible to use ice-bound (CO)-C-14 to reconstruct the historical flux of cosmic rays at Earth, without the transport and deposition uncertainties associated with Be-10 or the carbon cycle uncertainties affecting atmospheric (CO2)-C-14. The measurements will be sensitive to the cosmic-ray flux above the energy range most affected by solar modulation. We present estimates of the expected sensitivity of (CO)-C-14 in ice cores to the historical flux of Galactic cosmic rays, based on recent studies of (CO)-C-14 in polar ice.